The invention relates generally to the field of energy measurement devices used for monitoring energy flow in power systems. More specifically the invention relates to a measurement device that reduces the cost and complexity of the installation of the devices. More specifically the cost and installation of conduit, electrical wiring and communication wiring.
Intelligent Electronic Devices (IEDs) comprise, but are not limited to digital power/energy meters, protective relays, power quality measurement devices, fault recorders or other devices capable of interfacing to electric power lines and calculating at least one power parameter. Power parameters include, but are not limited to rms current, rms voltage, kW, kVAR, kVA, frequency, harmonics, kWh, kVARh, kVAh, power factor, symmetrical components, etc.
Current transformers are used to monitor the current flowing in power system conductors. Generally, current transformers consist of two types. The first type is the closed (toroidal or rectangular) type. The second type is the split core (clamp-on) type. The closed type consists of a toroidal or substantially rectangular section of magnetic material with a “window” or opening through the middle. The current transformers have at least one secondary transformer winding that is wound around the material and through the window. A primary winding normally consists of a power line in a power system passing through the window that forms a single transformer turn. The winding ratio of the transformer is then the ratio of the primary to secondary turns. Split-core type current transformers are of substantially the same shape as closed type current transformers with the addition of a split in the magnetic material such that the transformer can be placed around the primary winding without having to “thread” the primary winding through the window. This allows installation of the split-core type current transformer on power system cables without disconnecting them from their source or load.
Standard current transformers suffer from errors in both ratio and phase shift mainly due to the magnetization current required to excite the magnetic material of the core. These effects limit the accuracy of the current transformer and dynamic range of current the transformers are able to accurately sense. This is especially the case with split-core type current transformers due to the magnetic flux leakage caused by the split in the magnetic material.
An active or compensated current transformer circuit that corrects for such errors is described in U.S. Pat. No. 3,534,247 to Miljanic entitled “Current Transformer with Internal Error Compensation”. This circuit provides a substantially zero phase shift and zero ratio error current transformation using a compensation amplifier. Powering the compensation amplifier from an additional current transformer is included in the active current transformer circuit. The presence of a separate powering current transformer means that additional wires are present beyond those providing the secondary current. This may make the device undesirable for installation in locations such as switchgear cabinets due to the high voltages present.
A second active current transformation approach is described in U.S. Pat. No. 4,841,236 to Miljanic et al. entitled “Current Ratio Device”. This approach provides additional isolation over the approach of the U.S. Pat. No. 3,534,247 patent through the inclusion of an isolated additional secondary winding which provides advantages for uses in high accuracy metrology applications, but in general the accuracy of the approach of the U.S. Pat. No. 3,534,247 patent approach is more than adequate for most power system monitoring applications.
A self powered non-active current monitor for monitoring current in an electric power system is described in U.S. Pat. No. 6,018,700 to Edel entitled “Self-Powered Current Monitor”. This circuit provides power for amplification circuitry, a microprocessor, etc. that is derived from the power line that is being monitored. The circuit includes a burden reducing circuit that allows current monitoring to be performed with the same magnetic core that is powering the circuitry. The monitoring function of this circuit is not continuous or the burden of the power supply reduces the accuracy of the current transformation and therefore this approach is difficult to use with accurate advanced power monitoring devices that continuously sample the current waveform in order to provide accurate power calculations and power quality functionality.
Typically metering a particular point within an energy distribution system, such as an electrical distribution system, can be very costly and there are monetary and practical barriers to installing metering points. Some of these costs are external components such as potential transformers or current transformers, commissioning, mounting, conduit and installation costs.
A large installation cost associated with metering points is the installation of communication wires including conduits for the communication wires. The availability of existing communication, cost of extending those communications, and the labor involved in running the communication wires can be prohibitive when evaluating the benefit of adding a new metering point. Alternatively, a metering point may be installed without any communication. In this case, someone must physically go to the meter and read a display, record the energy values, and transport this information to a central system. This approach is prone to human error in addition over time it is an expensive communication method.
Another large installation cost associated with some metering points is the cost of providing an adequate power supply or separate power supply to the metering device. For example, a metering device may be monitoring at a point of a large voltage potential, while the device may be able to monitor the voltage potential, the control power required for the device must be at a significantly lower potential. Alternatively, a metering device may be monitoring a non-electrical quantity such as the output of a flow meter. There may not be a conventional power supply for the metering device accessible; in which case, additional installation expense is required to provide control power to the metering device.
A further installation cost associated with the physical mounting to the metering point. Typically metering points must be mounted into a dedicated measurement cabinet or measurement rack of some sort with a hole for a display or mounting screws to secure the metering device to the cabinet or rack.
Typically the advantages of installing metering points closer to the supply of the energy outweigh the barriers; however, as the energy travels closer down the energy distribution system to the consumer and finally to the load, the costs of metering points often outweighs the benefits. However, there is a desire and financial benefit to monitoring energy distribution at additional metering points; especially further down the energy distribution system towards the load. When more information is known of the energy used, more can be done to reduce energy usage. In addition, more energy metering points allow tenants to be billed by actual energy costs rather than an approximation such as a energy cost proportional to rented square footage.
When these costs are reduced, the financial reward of energy cost analysis outweighs the reduced cost barriers of metering closer to each energy load. These additional metering points can assist in creating a clearer representation of energy costs throughout a facility and that information can assist the facility in reducing its energy costs.